1. Field
The invention relates generally to holographic data storage media and systems, and more particularly to methods and systems for recording and/or reading holographic storage media.
2. Description of Related Art
Holographic data storage systems store information or data based on the concept of a signal beam interfering with a reference beam at a holographic storage medium. The interference of the signal beam and the reference beam creates a holographic representation, i.e., a hologram, of data elements as a pattern of varying refractive index and/or absorption imprinted in a volume of a storage or recording medium such as a photopolymer or photorefractive crystal. Combining a data-encoded signal beam, referred to as an object beam, with a reference beam can create the interference pattern at the storage medium. A spatial light modulator (SLM) or lithographic data mask, for example, may create the data-encoded signal beam. The interference pattern induces material alterations in the storage medium that generate the hologram.
The formation of the hologram in the storage medium is generally a function of the relative amplitudes and polarization states of, and phase differences between, the signal beam and the reference beam. The hologram is also dependent on the wavelengths and angles at which the signal beam and the reference beam are projected into the storage medium. After a hologram is created in the storage medium, projecting the reference beam into the storage medium interacts and reconstructs the original data-encoded signal beam. The reconstructed signal beam may be detected by using a detector, such as a CMOS photo-detector array or the like. The recovered data may then be decoded by the photo-detector array into the original encoded data.
A basic holographic system is illustrated in FIG. 1. The holographic storage system includes a light source 110, for example, a laser for providing a coherent beam of light. A beam splitter 114 is positioned to split the laser beam into an object beam and a reference beam. The object beam is directed to an SLM or data mask 116 where it is encoded with information as a two-dimensional image and directed to the recording storage medium 124 by mirror 118 and lens 120 where it interferes with the reference beam directed via mirror 130. A complex interference pattern is recorded in the storage medium 124 where the object beam and reference beam interact. After a first image or layer is recorded, the system may be modified to enable additional images to be recorded in storage medium 124. For example, by modifying the angle and/or wavelength of the reference beam, successive images may be recorded in the storage medium 124.
A particular image may be retrieved from recording medium 124 with a reference beam similar to the original reference beam used to store the image. The light is diffracted by storage medium 124 according to the stored hologram and the two-dimensional image that was stored in recording medium 124 is directed by lens 126 to photo-detector array 128.
Holographic Read Only Memory (Holographic ROM or HROM) storage media are well known. In the past, holographic information has been recorded in disc format HROM in an incremental manner by successively aligning different locations on the HROM with an object beam and a reference beam to record successive data bits. Different information can be recorded at each successive location by changing the information imparted through a spatial light modulator (SLM) or successive data masks, for example. U.S. Pat. No. 6,272,095, entitled, “Apparatus and Method for Storing and/or Reading Data on an Optical Disk,” by Liu et al. describes several examples of illustrative prior recording techniques, and is incorporated herein by reference in its entirety. Moreover, multiple holograms can be stacked in virtual image layers through wavelength multiplex, angle multiplex, shift multiplex, confocal multiplex, or other multiplex techniques, for example. Each hologram in a stack may comprise a page of information, where a “page” is a collection of bits or of pixel data stored together, e.g., as a 2048×2048 array or a 10×10 array. U.S. Pat. No. 6,322,933, entitled, “Volume Track Definition for Data Storage Media Used to Record Data by Selective Alteration of a Format Hologram,” by Daiber et al. describes several examples of illustrative prior volume recording techniques, and is incorporated herein by reference in its entirety. Additionally, another reference that describes recording techniques and the bitwise retrieval of an HROM includes “Holographic ROM System for High Speed Replication,” presented by Ernest Chuang et al. of Sony Corporation at the Optical Data Storage Conference, Jul. 8, 2002, in Hawaii, USA.
One shortcoming of such methods for the recording of holograms is that significant time can be required to incrementally record information on a location-by-location or bit-by-bit basis. Therefore, improved recording processes have been proposed in which an entire holographic image or “layer” of information is recorded simultaneously across an entire storage medium. An exemplary method includes shining a planewave beam through a transmissive optical medium, e.g., a transmissive lithographic data mask, encoded with information so as to create a planewave object/signal beam. The encoded planewave object beam illuminates one surface of the storage medium. A conical planewave reference beam, for example, may illuminate an opposite surface of the holographic storage medium. The object beam and the reference beam interfere within the holographic storage medium to create an information layer within the storage medium.
Readout of the data stored on the media may be subsequently achieved using the same reference beam used to record the data or by using a conjugate readout beam (i.e., similar to the original reference beam propagated in the opposite direction) to reconstruct a backwards-propagating signal beam that retraces the path of the original recording. The stored information is readout in a bitwise fashion, e.g., a bit at a time or a few bits at a time in a serial fashion as described in the Sony Corporation approach referenced above. A pickup lens of a system may focus a real image of the data, created using the readout beam, onto a suitable detector as the holographic medium and/or the drive translate and/or rotate with respect to each other.
One shortcoming of the recording methods described above is that information typically is readout a bit at a time or a few bits at a time in a serial fashion. Thus, there is a need for recording holographic storage media that more readily permits relatively higher, parallel readout rates. Specifically, there is a need for parallel readout of at least a page of information at a time, e.g., many bits in parallel. In addition, the bit-by-bit readout architecture generally limits the implementation to spinning disks in order to get reasonable transfer rates. There is a need for page-wise readout that may allow for new more compact, portable formats such as storage cards to be practical. Another shortcoming of the prior proposed recording methods includes the alignment and time necessary to align and record multiple data masks in holographic storage media, e.g., HROM media.